1. Field of the Invention
The present invention relates to a solid-state imaging device and a method for manufacturing the same, and particularly relates to what is called a solid-state imaging device of a back-illuminated type and a method for manufacturing the same.
2. Description of the Related Art
With a high density integration of a semiconductor device, a transistor and other semiconductor elements have been tried to be more miniaturized and to raise mounting density more than before.
Therefore, in a CMOS image sensor (CMOS type solid-state imaging device), it is required that a pixel is made minute and an element is made to be highly integrated.
However, in a conventional CMOS image sensor in which light detection is performed by irradiating a photo sensor portion with light from a lens formed on a wiring portion through the space between wiring layers, the photo sensor portion can not be irradiated with an enough amount of light, because an eclipse of incident light due to the obstacle such as a wiring layer occurs and an aperture ratio of the photo sensor portion is made small in accordance with the high density integration of device to make the pixel become minute. Accordingly, such problems as lower sensitivity and more shading may occur.
Therefore, the photo sensor portion is irradiated with light from the rear side (opposite side to the wiring portion), so that an effective 100% ratio of aperture can be achieved and sensitivity can be sufficiently raised without an influence of an obstacle such as a wiring layer.
Accordingly, a CMOS image sensor in which the photo sensor portion is irradiated with light from the rear-surface side (opposite side to the wiring portion), what is called a back-illuminated type CMOS image sensor, has been developed.
Then, in the back-illuminated type CMOS image sensor, it is considered that a silicon layer of the photo sensor portion is made thin to obtain higher sensitivity (refer to, for example, patent document 1 or patent document 2).
FIG. 1 is a vertically sectional view schematically showing a back-illuminated type CMOS image sensor to which the above described construction is applied.
In a MOS sensor 100, since a silicon substrate 101 including photo sensor portions 102 is made thin, sensitivity to incident light L can be raised.
Further, since the incident light L is not blocked even if wiring layers 104 are formed on the photo sensor portions 102, flexibility in layout of the wiring layers 104 can be obtained. Accordingly, the high density integration of an element can be obtained by forming the wiring layer 104 to be multi-layered and by minimizing an area of a pixel.
A method for manufacturing the back-illuminated type image sensor having such a thin silicon layer is considered, in which, for example, after a photo diode of a photo sensor portion is formed in a silicon substrate, the silicon substrate is ground from the back thereof to be thin.                Patent document 1: Japanese Published Patent Application No. H6-77461 (FIG. 3)        Patent document 2: Japanese Published Patent Application No. H6-283702 (FIG. 2)        
However, in the above-described manufacturing method, an interface of the silicon substrate after grinding the back surface becomes electrically unstable. Also, a mechanical damage may affect the silicon substrate.
As a result, dark current is caused due to the above problems.
Hence, an image sensor of the back-illuminated type having the above-described structure has a limitation in use such as requiring a cooling process.
Furthermore, light absorption depends upon the thickness of a silicon layer; however, since the thickness is made thin by grinding, control for the thickness of the silicon layer (silicon substrate) of the photo sensor portion becomes deteriorated, so that the spectroscopic characteristic as the sensor tends to disperse.
As a result, the manufacturing yield ratio becomes deteriorated, and the cost may increase because of such a problem.
Therefore, the back-illuminated type structure has been used only for limited purposes, though the sensitivity of the photo sensor portion could be improved.
In addition, a conventional image sensor of the back-illuminated type structure has several ten micro-meter of the thickness of a silicon layer in which the photo sensor portion performing the photo-electric conversion is formed; and when a pixel is miniaturized, the electric-charge of a signal diffuses between adjacent pixels to generate mixed color.
Accordingly, it is difficult to realize a minute pixel.